Q420D
Low-alloy high-strength structural steel (HSLA) · GB/T 1591-2018 — High strength low alloy structural steels (current) · GB/T 1591-2008 — High strength low alloy structural steels (withdrawn legacy edition)
Q420D denotes a 420 MPa-class HSLA structural steel with D-quality low-temperature toughness, but it is not a complete current GB/T 1591-2018 designation. The 2018 edition explicitly incorporates delivery condition into the grade: legacy Q420D normally becomes Q420ND for normalized/normalizing-rolled material or Q420MD for TMCP material. These routes have different chemistry, carbon-equivalent limits, thickness ranges and tensile requirements, so they must not be treated as one interchangeable property set. Existing drawings, certificates or purchase orders stating only “Q420D to GB/T 1591” should have the standard edition and delivery condition clarified before acceptance or substitution.
- Q420D alone is incomplete under GB/T 1591-2018Current designation issue · Use Q420ND or Q420MD according to the required delivery condition.
- 420 MPa minimum upper yield strength at the smallest specified thicknessesNominal strength class · The required minimum decreases as thickness or diameter increases.
- Charpy V-notch testing at −20 °C by defaultD-quality toughness · For Q420ND and Q420MD, full-size specimens require at least 40 J longitudinal or 20 J transverse at −20 °C.
- N or MCurrent delivery routes · N means normalized/normalizing rolled; M means thermomechanically processed.
- CEV ≤0.43–0.47%; Pcm ≤0.20%TMCP weldability control · Limits for Q420MD depend on thickness; Pcm may be used under the conditions stated by the standard.
- L04204Legacy identifier · Associated with Q420D in GB/T 1591-2008; it does not remove the need to identify the applicable standard edition and condition.
Overview
- Designation system
- Chinese GB/T yield-strength designation
- Product forms
- Steel plate and strip, Hot-rolled sections, Steel bars, Billets, where ordered under the standard
- Condition
- Legacy Q420D: delivery condition must be established from the purchase specification and certificate, Q420ND: normalized or normalizing rolled, Q420MD: thermomechanically processed (TMCP), optionally followed by tempering where permitted
- Density
- 7850 kg/m³ (Conventional engineering value for structural steel; not a GB/T 1591 grade requirement.)
Designation and standard-edition control
In the older GB/T 1591-2008 system, Q420D comprised Q for yield strength, 420 for the nominal minimum yield-strength class in MPa, and D for quality grade. Delivery condition was not embedded in the short designation. GB/T 1591-2018 changed the designation structure by adding a delivery-condition code before the quality letter. Consequently, a normalized or normalizing-rolled product is Q420ND, while a thermomechanically processed product is Q420MD.
| Document reference | Technically appropriate interpretation | Action |
|---|---|---|
| GB/T 1591-2008, Q420D | Valid legacy designation | Apply the 2008 requirements and verify the actual delivery condition from the order and certificate. |
| GB/T 1591-2018, Q420D | Incomplete/nonconforming designation syntax | Clarify whether Q420ND or Q420MD was intended. |
| GB/T 1591 without an edition | Ambiguous | Establish the contractual edition; do not select properties from whichever edition is more convenient. |
| Certificate marked Q420ND | Normalized or normalizing-rolled D-quality steel | Apply the Q420N chemistry, CEV and tensile-property tables. |
| Certificate marked Q420MD | TMCP D-quality steel | Apply the Q420M chemistry, CEV/Pcm and tensile-property tables. |
The N and M routes are not merely alternative spellings. Their rolling histories, chemistry limits, weldability controls, dimensional coverage and response to subsequent heating differ.
Current normalized route: Q420ND
Q420ND is the current designation for D-quality material delivered normalized or normalizing rolled. Normalizing rolling is controlled so that the resulting condition and mechanical properties are comparable to those obtained by normalizing. This route is the direct current-standard mapping for legacy Q420D that was supplied normalized or normalizing rolled.
| Element or control | Requirement, mass % | Comment |
|---|---|---|
| C | ≤0.20 | Maximum |
| Si | ≤0.60 | Maximum |
| Mn | 1.00–1.70 | Specified range |
| P | ≤0.030 | D quality |
| S | ≤0.025 | D quality |
| Nb | 0.01–0.05 | Grain-refining/microalloy element |
| V | 0.01–0.20 | Grain-refining/microalloy element |
| Ti | 0.006–0.05 | Grain-refining/microalloy element |
| Cr / Ni / Cu / Mo | ≤0.30 / ≤0.80 / ≤0.40 / ≤0.10 | Individual maxima |
| Acid-soluble Al | ≥0.015 | Total Al may be used subject to the standard's conditions |
At least one specified grain-refining element is to be intentionally present at or above its applicable lower limit. Combination and nitrogen-control provisions in the standard also apply.
| Nominal thickness or diameter, mm | Minimum ReH, MPa | Rm, MPa | Minimum elongation A, % |
|---|---|---|---|
| ≤16 | 420 | 520–680 | 19 |
| >16–40 | 400 | 520–680 | 19 |
| >40–63 | 390 | 500–650 | 19 |
| >63–80 | 370 | 500–650 | 18 |
| >80–100 | 360 | 500–650 | 18 |
| >100–150 | 340 | 500–650 | 18 |
| >150–200 | 330 | 500–650 | 18 |
| >200–250 | 320 | 500–650 | 18 |
ReH is the upper yield strength; Rp0.2 may be used when a distinct yield phenomenon is absent. Tensile ranges and applicability must be read with the standard's product-form and sampling provisions.
| Nominal thickness or diameter, mm | Maximum CEV, % |
|---|---|
| ≤63 | 0.48 |
| >63–100 | 0.50 |
| >100–250 | 0.52 |
| >250–400 | By agreement |
The CEV limits apply to Q420N quality grades covered by the table and are based on ladle analysis.
Current TMCP route: Q420MD
Q420MD is D-quality Q420 steel produced by thermomechanical processing. TMCP combines controlled deformation and temperature control, potentially including accelerated cooling, to obtain properties that cannot necessarily be reproduced by conventional heat treatment. This route generally uses lower carbon and tighter weldability controls than Q420ND, but its properties are more sensitive to later high-temperature processing.
| Element or control | Requirement, mass % | Comment |
|---|---|---|
| C | ≤0.16 | Maximum for Q420M |
| Si | ≤0.60 | Maximum |
| Mn | ≤1.70 | Maximum |
| P | ≤0.030 | D quality |
| S | ≤0.025 | D quality |
| Nb | 0.01–0.05 | Specified range |
| V | 0.01–0.12 | Specified range |
| Ti | 0.006–0.05 | Specified range |
| Cr / Ni / Cu / Mo | ≤0.30 / ≤0.80 / ≤0.40 / ≤0.20 | Individual maxima |
| Acid-soluble Al | ≥0.015 | Subject to the standard's alternative aluminium and microalloy provisions |
The standard permits alloy design within these limits; a commercial heat need not contain every listed alloy at its maximum or throughout its stated range unless the applicable footnotes require intentional addition.
| Nominal thickness or diameter, mm | Minimum ReH, MPa | Rm, MPa | Minimum elongation A, % |
|---|---|---|---|
| ≤16 | 420 | 520–680 | 19 |
| >16–40 | 400 | 520–680 | 19 |
| >40–63 | 390 | 500–660 | 19 |
| >63–80 | 380 | 480–640 | 19 |
| >80–100 | 370 | 470–630 | 19 |
| >100–120 | 365 | 460–620 | 19 |
For bars, the standard extends certain TMCP provisions to 150 mm; the precise product-form footnotes and property table must be checked rather than assuming the plate limits apply.
| Nominal thickness or diameter, mm | Maximum CEV, % | Maximum Pcm, % |
|---|---|---|
| ≤16 | 0.43 | 0.20 |
| >16–40 | 0.45 | 0.20 |
| >40–63 | 0.46 | 0.20 |
| >63–120 | 0.47 | 0.20 |
| >120–150 | 0.47 | 0.20 |
The >120–150 mm range is limited to bars. Where carbon is not greater than 0.12%, the standard permits Pcm to be used in place of CEV for weldability assessment; the ordering agreement should state which index is required.
Toughness and test applicability
| Test temperature | Minimum longitudinal KV2 | Minimum transverse KV2 | Status |
|---|---|---|---|
| 20 °C | 55 J | 31 J | Values listed for D quality |
| 0 °C | 47 J | 27 J | Values listed for D quality |
| −20 °C | 40 J | 20 J | Default D-quality test temperature when the purchaser does not specify otherwise |
| −30 °C | 27 J | Not specified by the option | Optional D-quality test by purchaser specification |
The standard normally uses longitudinal impact specimens; transverse specimens may be agreed. Impact testing applies from nominal thickness 6 mm or diameter 12 mm. Full-size specimens are 10 × 10 × 55 mm.
If the product cannot provide a full-size Charpy specimen, 10 × 7.5 mm or 10 × 5 mm sub-size specimens are used. Their required absorbed energies are respectively 75% and 50% of the full-size value, with the largest practicable specimen preferred. A certificate showing an impact value without temperature, orientation and specimen size is therefore insufficient for verification.
Legacy Q420D under GB/T 1591-2008
Q420D was a valid designation in GB/T 1591-2008. It specified the strength class and D quality but did not encode the delivery condition. The 2018 standard's informative comparison annex maps legacy normalized/normalizing-rolled Q420D to Q420ND and legacy TMCP Q420D to Q420MD. This is a route-dependent succession, not a basis for combining the two current property sets.
| Requirement | Legacy value |
|---|---|
| Key ladle-analysis maxima | C 0.20%; Si 0.50%; Mn 1.70%; P 0.030%; S 0.025% |
| Yield strength, t ≤16 mm | ≥420 MPa |
| Yield strength, >16–40 mm | ≥400 MPa |
| Yield strength, >40–63 mm | ≥380 MPa |
| Yield strength, >63–100 mm | ≥360 MPa |
| Yield strength, >100–150 mm | ≥340 MPa |
| Tensile strength, t ≤100 mm | 520–680 MPa |
| Tensile strength, >100–150 mm | 500–650 MPa |
| Minimum elongation | 19% through 40 mm; 18% above 40 through 150 mm |
| D-quality Charpy test | Nominally −20 °C under the legacy quality-grade system |
These are legacy requirements and must not be represented as the current requirements for both Q420ND and Q420MD.
Welding, forming and thermal processing
CEV or Pcm is a compositional indicator, not a complete welding procedure. Preheat and interpass temperature must be selected using the actual certificate chemistry, product thickness, joint restraint, heat input, consumable hydrogen level and applicable fabrication code. Qualification of a welding procedure remains necessary where required by the construction standard.
For thermomechanically processed material, the standard warns that hot forming or post-weld heat treatment above 580 °C may reduce strength. Such processing should not be specified for Q420MD without producer guidance and procedure qualification. This warning should not automatically be transferred to Q420ND as though the two delivery conditions respond identically.
A 180° bend test is an optional order requirement under GB/T 1591-2018. The prescribed former diameter is 2a for nominal thickness or diameter up to 16 mm and 3a for over 16 through 100 mm, where a is the specimen thickness or diameter. Plate and strip at least 600 mm wide use transverse tensile specimens; other products generally use longitudinal specimens.
Ordering and acceptance checklist
| Item | Why it matters |
|---|---|
| Standard edition | Determines whether Q420D is a valid complete designation and which tables apply. |
| Full current designation | State Q420ND or Q420MD; do not order current material as Q420D alone. |
| Product form and dimensions | Strength, tensile range, test orientation and dimensional coverage depend on form and thickness or diameter. |
| Delivery condition | Controls chemistry, weldability limits and response to subsequent heating. |
| Impact temperature and orientation | D quality defaults to −20 °C, but orientation and optional lower-temperature testing may need to be stated. |
| CEV or Pcm reporting | Useful for welding assessment and should be requested where project controls require it. |
| Z-direction properties | For through-thickness ductility, append the required GB/T 5313 class, such as Z25, and include it in the order. |
| Nondestructive examination | Not automatic; method, acceptance level and extent must be agreed. |
| Surface condition and dimensional tolerances | Invoke the applicable plate, strip, section or bar standard and required tolerance class. |
| Inspection certificate | Check heat identity, grade, condition, dimensions, chemistry, CEV/Pcm where applicable, tensile results and Charpy temperature/orientation/specimen size. |
For an existing item marked only Q420D, the safest identification sequence is: determine the cited GB/T 1591 edition, inspect the mill certificate for delivery condition, confirm product form and thickness, and then compare the complete chemistry, tensile and impact requirements. Grade-name similarity alone is not adequate for acceptance.
Sources
- GB/T 1591-2018 — High strength low alloy structural steels: official standard recordState Administration for Market Regulation / Standardization Administration of China
- GB/T 1591-2018 — High strength low alloy structural steels: standard textChina Baowu / Baosteel
- GB/T 1591-2008 — High strength low alloy structural steels: official withdrawn-standard recordState Administration for Market Regulation / Standardization Administration of China
- GB/T 1591-2008 — High Strength Low Alloy Structural Steels, English textSteel Building Technology
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